Strict regulation of cell cycle progression, including differentiation, senescence, and cell death, is critical for the proper development and maintenance of tissues. Dysfunction of the regulation of these processes can result in devastating pathological conditions including, for example, cancer.
One aspect of a normal cell cycle includes biochemically regulated cell death, also known as apoptosis. Regulators of apoptosis, both positive and negative, have been identified. For example, the protein known as Bcl-2 counters a variety of apoptotic stimuli (Vaux et al., 1988; Strasser et al., 1991; Garcia et al., 1992). CED-9, the homolog of Bcl-2 in the nematode Caenorhabditis elegans, is found to repress apoptosis in cells that are normally expected to die during the nematode's development. Studies involving transgenic worms expressing Bcl-2 indicate that Bcl-2 can substitute for CED-9 functionally in preventing at least some cell death in these nematodes. (Vaux et al., 1992; Hengartner and Horvitz, 1994).
Many Bcl-2 related proteins share homology within two conserved regions: Bcl-2 homology domains 1 and 2 (BH1 and BH2) (Williams and Smith, 1993; Yin et al., 1994). These proteins include Bax, Bcl-x.sub.L, Mcl-1, and A1 (Olwai et al., 1993; Boise et al., 1993; Kozopas et al., 1993; Lin et al., 1993). Several of these proteins are cell death regulators; for example, Bcl-x.sub.L represses apoptosis, while its short form, Bcl-x.sub.S, favors cell death. Additionally, Bax in excess interferes with the ability of Bcl-2 to repress apoptosis. Bax homodimerizes and also heterodimerizes with Bcl-2 (Oltvai et al., 1993). Single amino acid substitutions have been found to disrupt Bcl-2-Bax heterodimers, but not Bcl-2-Bcl-2 homodimers. Bcl-2 mutants that did not complex with Bax could no longer repress apoptosis (Yin et al., 1994). These data suggest that the cell cycle regulatory functions of these proteins occur at least partially through protein-protein interactions.
Bad, the Bcl-x.sub.L /Bcl-2-associated death promoter homolog, is conserved within the BH1 and BH2 domains (Yang, E., J. Zha, J. Jockel, L. H. Boise, C. B. Thompson, S. J. Korsmeyer [1995] Cell 80:285-291). Bad has been shown to heterodimerize with Bcl-xL and Bcl-2, but not with other related proteins. One way in which Bad promotes mammalian cell death is by displacing Bax from Bcl-x.sub.L as it heterodimerizes with Bcl-x.sub.L.